EGR is a known method for reducing NOx emissions in internal combustion engines. A conventional EGR system works by taking a by-pass stream of engine exhaust gas from an engine exhaust manifold and pressurizing the exhaust gas a desired amount for injection into the engine's induction system, mixing with the intake air and combustion fuel mixture, and for subsequent combustion. A control valve is used within the EGR system to regulate the amount of exhaust gas that is routed to the engine induction system based on engine demand. The process of recirculating the exhaust gas insures that partially oxidized NOx become fully oxidized, thereby reducing smog producing partially-oxidized NOx emissions. Accordingly, such a conventional EGR system typically comprises exhaust by-pass tubing, related plumbing and manifolding, an engine crankshaft-driven EGR pump, and an EGR control valve, all of which are ancillary components that are attached to the engine.
A disadvantage of such conventional EGR systems is that they require the use of ancillary moving components, e.g., a pump and control valve, that are capable of failing or otherwise not performing properly, thereby interfering with the effective reduction of NOx. Additionally, these components must be attached externally the engine, thereby occupying space within an engine compartment.
It is, therefore, desirable that an EGR system be constructed that does not depend on the use of such external and ancillary moving parts. It is also desirable that such EGR system provide a level of NOx reduction that is equal to or better than that provided by conventional EGR systems.
Turbochargers for gasoline and diesel internal combustion engines are known devices used in the art for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed via an exhaust manifold or exhaust pipe into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing.
The exhaust gas routed to such turbocharger is a by-pass stream taken from the combined exhaust stream generated by the engine, e.g., from an exhaust manifold or exhaust pipe that combines the different exhaust gas streams leaving each engine cylinder. Accordingly, the exhaust gas routed to such a turbocharger passes to the turbocharger at a substantially unpulsed or continuous volumetric flow rate. Of course the amount of exhaust gas flow routed to the turbocharger will increase with increasing engine speed or rpm. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the exhaust housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
The amount by which the intake air is boosted or pressurized is controlled by regulating the amount of exhaust gas that is passed through the turbine housing by a wastegate valve. The wastegate valve is actuated, during turbocharger operation when the boost pressure is approaching a maximum desired pressure, to divert an amount of exhaust gas away from the turbocharger turbine housing to reduce the rotational speed of the turbine and, thereby reduce both the rotational speed of the air compressor and the amount by which the intake air is pressurized.
The above-described conventional turbocharger system is driven by a substantially continuous pressure exhaust by-pass stream from the engine exhaust. In such a system, the backpressure of the turbocharger can cause the exhaust pressure within the upstream exhaust system to be increased, ultimately robbing pumping energy from the engine crankshaft. Thus, although such a turbocharger functions to increase the boost pressure and combustion energy within the engine, it does so at a cost of increased backpressure.
It is, therefore, desirable that a turbocharger system be constructed that provides increased intake air boost pressure without significantly increasing the exhaust backpressure within the engine. It is also desired that such turbocharger system be capable of maintaining a positive pressure difference across an engine cylinder head at all operating points to both provide improved engine output and improved overall pumping efficiency.